Method and system for synchronization in communication system

ABSTRACT

A method for uplink synchronization of a first transceiver and a second transceiver in a multi-user cellular communication system having communication resources divided into communication channels. The method includes the following steps. A first signature sequence is transmitted from the second transceiver to the first transceiver, where the signature sequence is selected from a first set of signature sequences. In the first transceiver, the received signal is correlated with at least one signature sequence from a second set of signature sequences to estimate the time of arrival of the signature sequence to synchronize transmission between the second transceiver and the first transceiver. In transmitting the first signature sequence from the second transceiver to the first transceiver, the first signature sequence constitutes at least part of a zero-correlation zone sequence.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of PCT/CN2006/000077 filed Jan. 18,2006, which is incorporated by reference in its entirety herein. The PCTapplication published in English as WO2007/082409 A1.

FIELD OF THE INVENTION

The disclosure relates to the field of radio communication systems, andin particular to a method for uplink synchronization of a base stationand a mobile terminal in a multi-user cellular communication system.

BACKGROUND OF THE INVENTION

In most mobile communication systems of today, there are specificrequirements regarding synchronization of a base station and a mobileterminal in order to secure a correct data transmission. Examples ofsuch systems are the Universal Terrestrial Radio Access (UTRA) andEvolved UTRA.

In Evolved UTRA, Single-Carrier Frequency Division Multiple Access(SC-FDMA) may be used as multiple access scheme for the uplinkcommunication. The transmission scheme of SC-FDMA is the so-calledDiscrete Fourier Transform-spread Orthogonal Frequency DomainMultiplexing (DFT-spread OFDM), which can be seen as OFDM withpre-coding. Whereas OFDM, which produces a multi-carrier signal, has ahigh peak-to-average ratio (PAPR), the DFT pre-coding gives asingle-carrier signal with lower PAPR. The low PAPR serves to extend thecoverage and to reduce the battery drain in the mobile.

In DFT-spread OFDM, cyclic prefix is used to achieve equalization in thefrequency domain. However, a requirement for successful equalization inDFT-spread OFDM, as well as in OFDM, is that the signals transmittedfrom all mobile terminals in the cell are synchronized in such a mannerthat the delay spread of the signal plus the spread in the time ofarrival is less than the duration of the cyclic prefix. Therefore, it isrequired that each transmitting mobile terminal is synchronized towithin a fraction of the duration of the cyclic prefix before it cantransmit data.

In Evolved UTRA, synchronization is performed both in uplink anddownlink. In one step of the synchronization, downlink synchronization,the mobile terminal synchronizes (or locks) to the carrier frequency andthe frame timing of the base station. This synchronization, however, isnot sufficient to ensure that the base station can properly receive thesignals from the mobile terminal, since mobile terminals may be locatedat various distances relative to the base station.

Consequently, further synchronization, uplink synchronization, is neededsince the distance between a base station and a mobile terminal, andhence the round trip time, is in general unknown.

In Evolved UTRA, a random access channel (RACH) supports uplinksynchronization of the mobile terminals. RACH in Evolved UTRA iscontention-based, i.e. any mobile terminal within the cell may transmiton the resource allocated to RACH. Consequently, several mobileterminals may attempt to transmit synchronization signalssimultaneously, and in order to reduce the risk that the base stationfails to distinguish signals from different mobile terminals, a set ofsignature sequences is provided, wherein each mobile terminal randomlyselects one signature sequence.

In UTRA and Evolved UTRA a binary pseudo-random sequence generated by ashift register is modulated by 16-bit Hadamard sequences to producethese signature sequences. Even though these signature sequences in manyinstances provide good correlation properties, there still exists a needfor enhanced detection capabilities to detect a specific signature inpresence of other simultaneous signatures, especially at low SIR values.

SUMMARY OF THE INVENTION

It is an object of the disclosure to provide a method and a system foruplink synchronization in a multi-user cellular communication system,which has enhanced capabilities to detect a single signature in presenceof a number of other simultaneous signatures, especially at lowsignal-to-interference ratio (SIR) values, as compared to the knownprior art.

In accordance with the present disclosure, a signature sequence istransmitted from a second transceiver to a first transceiver. Thesignature sequence is selected from a first set of signature sequences,and the received signal is correlated with at least one signaturesequence in the first transceiver to estimate the time of arrival tosynchronize transmission between the second transceiver and the firsttransceiver. The invention is characterized by a signature sequencecomprising at least part of a zero-correlation zone sequence. Thesignature sequence may be selected from a group of signature sequences.

The invention provides the advantage that, apart for maintainingfavourable features of prior art signature sequences,—such as goodautocorrelation properties for allowing accurate timing estimation, goodcross-correlation properties to allow for accurate timing estimation ofdifferent simultaneous and partially synchronized signature sequences,and a small peak-to average power ratio,—zero, or substantially zerocross-correlation for synchronous and simultaneous signature sequencesis achieved, which substantially improves the detection probability of aparticular signature sequence since the sequences are easilydistinguished from each other. The improved detection capabilitiesprovide the further advantage that in situations with more than onesimultaneously transmitted signature sequence, less retransmissions haveto occur due to missed detections, and, accordingly, system resourcesare more efficiently used. Further, as it is becoming more and moreimportant to obtain a fast access to the network and to be able toquickly transmit data using high power, the improved detectioncapabilities allow faster detection of a specific mobile terminal thatwishes to transmit data, which also facilitates interoperability withthe IP protocol.

The use of signature sequences according to the present inventionfurther has the advantage that, even if the signal level of onesignature sequence is strong while the signal level of a substantiallysimultaneous signature sequence is considerably weaker, e.g., due todistance, shadowing or (perhaps most probably) fast fading, theprobability of a correct detection is substantially improved.

The zero-correlation zone of said first signature sequence may be of alength such that it substantially corresponds to the maximum expecteddelay of a transmission from the second transceiver to the firsttransceiver. Further, the received signal may be correlated with atleast one signature sequence for a predetermined number of delays of thesignal, e.g., corresponding to the maximum expected delay. The delay maybe determined using the cell size. This approach has the advantage thata desired length of the zero-correlation zone may be obtained, wherebythe number of signature sequences may be varied to provide the requiredzero-correlation zone length. The more sequences, the shorterzero-correlation zone.

A set of matched filters may be used in the first transceiver tocorrelate the received signal with at least one signature sequence oreach signature sequence in a group of signature sequences for apredetermined number of delays of the signals, whereupon a peak outputfrom each matched filter is detected, and after which the detected peakoutput from each filter is used to estimate the time of arrival tosynchronize the transmission from the second transceiver. This has theadvantage that the correlation may be performed in a simple manner.

The signature sequences may be taken from a set of GeneralizedChirp-Like sequences obtained by modulating a Zadoff-Chu sequence withan orthogonal set of complex sequences. For example, the orthogonal setof modulating sequences is a set of rows and/or columns of a discreteFourier transform matrix, or a set of rows and/or columns in a Hadamardmatrix. This has the advantage that the signature sequences may beaccomplished in a simple manner.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more readily apparent from the DetailedDescription of the Invention, which proceeds with reference to thedrawings, in which:

FIG. 1 shows a conventional DFT-spread OFDM transmitter structure forsynchronized data transmission;

FIG. 2 shows absolute values of the autocorrelation andcross-correlation functions of exemplary signature sequences accordingto the present invention;

FIG. 3 shows a magnified portion of the graph in FIG. 2 in greaterdetail;

FIG. 4 shows the probability of missed detection for one transmittedsequence according to the present invention; and

FIG. 5 shows the probability of missed detection for a transmittedsequence in the presence of one or more other transmitted sequencesaccording to the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention will now be described more in detail in relationto a communication system employing DFT-spread OFDM.

In FIG. 1 is shown a basic transmitter 10 for DFT-spread OFDM. Blocks ofM complex modulated symbols x_(n), n=0, 1, . . . , M−1, are transformedby a DFT 11 which results in M coefficients X_(k):

$\begin{matrix}{{X_{k} = {\sum\limits_{n = 0}^{M - 1}{x_{n}{\mathbb{e}}^{{- j}\; 2\;\pi\frac{nk}{M}}}}},{k = 0},1,\ldots\mspace{14mu},{M - 1.}} & (1)\end{matrix}$

The output from the DFT is mapped by a sub-carrier mapping module 12 onequidistant sub-carriers l_(k)=l₀+kL, where l₀ is a frequency offset,and L is an integer larger than or equal to 1. All other inputs to theN-point Inverse Discrete Fourier Transform (IDFT) are set to zero.

The output of the IDFT 13, y_(n), is given by

$\begin{matrix}{{y_{n} = {\frac{1}{M}{\sum\limits_{k = 0}^{M - 1}{X_{k}{\mathbb{e}}^{j\; 2\;\pi\frac{{nl}_{k}}{N}}}}}},{n = 0},1,\ldots\mspace{14mu},{N - 1}} & (2)\end{matrix}$Finally, to avoid inter-symbol interference (ISI) and inter-channelinterference (ICI), a cyclic prefix inserter 14 inserts a cyclic prefix,i.e., a copy of the last portion of each OFDM symbol is inserted beforethe beginning of the same symbol. A time window may be applied after thecyclic prefix to reduce out-of-band emissions.

The cyclic prefix enables equalization in the frequency domain. However,a requirement for successful equalization in DFT-spread OFDM, as well asOFDM, is synchronization of the transmitted signals from all mobileterminals in a cell so that the delay spread of the signal plus thespread in the time of arrival is less than the duration of the cyclicprefix. It is therefore required that the mobile terminal issynchronized to within a fraction of the duration of the cyclic prefixbefore it can transmit data.

As stated above, in a first synchronization step in a DFT-spread OFDMsystem, the mobile terminal uses the carrier frequency and frame timingof the base station to perform synchronization. Although thissynchronization step ensures that the downlink synchronized mobile canreceive the signals from the base station, further synchronization isneeded to compensate for the, generally unknown, distance between themobile terminal and the base station so as to ensure that the basestation can properly receive the signals from the mobile terminal. Amobile terminal far away from the base station will receive downlinksignals with a larger delay than a mobile terminal close to the basestation, and the transmitted signals in uplink will take longer time topropagate to the base station for this mobile terminal, as compared tosignals from a mobile terminal closer to the base station. Once the basestation has estimated the time it will take for a signal transmittedfrom the mobile terminal to reach the base station, the base station maytransmit a command to the mobile terminal to adjust its transmit timingso that transmissions from various mobile terminals arrive at the basestation at desired points in time.

An important aspect of the second step of synchronization is that themobile terminals have already synchronized the reception of the downlinksignal and that all variations in time of arrival at the base station ofthe signals transmitted from the mobile terminals are due to thedifferent round-trip times. Since the cell size is known, the range oftime of arrivals is known a priori in the base station.

In Enhanced UTRA, the random access channel (RACH) in uplink supportsuplink synchronization of mobile terminals. It is mapped onto certainresources in time (access slots) and frequency. In each access slotthere should be a guard interval, so that all the transmitted signalsarrive within the allocated time and do not interfere with datatransmissions no matter where the transmitting mobile terminal islocated in the cell. Since the RACH in Evolved UTRA is contention-based,i.e. any mobile terminal within the cell may transmit on thetime-frequency resource allocated to RACH, more than one mobile terminalmay simultaneously, or substantially simultaneously, attempt to transmitsynchronization signals. In order to reduce the risk that the basestation fails to distinguish the signals from the different mobileterminals, a set of signature sequences is used, wherein each mobileterminal, normally in a random fashion, selects one signature sequenceout of the set of signature sequences.

Since successful detection of the signature sequence is necessary forthe mobile terminal to access the network, it is important that thetransmitted signature sequence requires a low power amplifier back-offto allow for high average transmit power and hence good coverage.

The signature sequences in uplink should have the following properties:

good autocorrelation properties to allow for accurate timing estimation,

good cross-correlation properties to allow for accurate timingestimation of different simultaneous and partially synchronized (i.e.downlink synchronized) signature sequences, wherein the phase differenceis limited by the maximum round-trip time in the cell,

zero cross-correlation for synchronous and simultaneous signaturesequences, and

a small peak-to-average power ratio.

These properties are satisfied to a large extent by the RACH signaturesin UTRA used today, and, at least partially, these also constitute thecurrent suggestion for Evolved UTRA. In UTRA, a binary pseudo-randomsequence generated by a shift register is modulated by 16-bit Hadamardsequences to produce the signature sequences. Further, rotation of thesignal constellation is applied to reduce the PAPR of the signal.

The modulation using Hadamard sequences allows for complexity reductionin the receiver: For each delay, the received signal is multipliedelement-wise with the complex conjugate of the pseudo-random scramblingsequence. Every 16^(th) sample is summed to produce a vector of 16elements. Finally, the Hadamard sequences are correlated with thereceived vector to produce correlation outputs of the signaturesequences.

However, some of the properties of these known signature sequences, suchas mutual cross-correlation, i.e. the detection probability of a singlesignature in presence of one or more other simultaneous signatures couldbe better, especially at low SIR values.

According to the present invention, the above problem is overcome byusing zero-correlation zone sequences, i.e., a downlink synchronizedmobile terminal transmits a signal, which is a signature sequence from aset of zero-correlation zone sequences.

A set of M sequences {d_(x)(k)}, x=0, 1, . . . , M−1, k=0, 1, . . . ,N−1, of length N is said to be a set of zero-correlation zone sequencesif all the sequences in the set satisfy the following autocorrelationand cross-correlation properties:

The periodic autocorrelation function

$\sum\limits_{k = 0}^{N - 1}{{d_{x}(k)}{d_{x}^{*}\left( {\left( {k + p} \right)\mspace{11mu}{mod}\; N} \right)}}$is zero for all p such that 0<|p|≦T and the periodic cross-correlationfunction

$\sum\limits_{k = 0}^{N - 1}{{d_{x}(k)}{d_{y}^{*}\left( {\left( {k + p} \right)\mspace{11mu}{mod}\; N} \right)}}$is zero for all p such that |p|≦T (including p=0). T is the length ofthe zero-correlation zone.

In an exemplary embodiment of the invention, the set of zero correlationzone sequences is constructed by using Generalized Chirp-Like (GCL)sequences. A GCL sequence {c(k)} is defined asc(k)=a(k)b(k mod m), k=0, 1, . . . , N−1.  (3)

where N=sm², s and m are positive integers, {b(k)} is any sequence of mcomplex numbers of unit magnitude, and {a(k)} is the Zadoff-Chu sequence

${a(k)} = \left\{ \begin{matrix}{W_{N}^{{k^{2}/2} + {qk}},} & {N\mspace{14mu}{even}} \\{W_{N}^{{{k{({k + 1})}}/2} + {qk}},} & {N\mspace{14mu}{odd}}\end{matrix} \right.$k=0, 1, . . . , N−1, q is any integer, (4)

where W_(N)=exp(−j2πr/N) and r is relatively prime to N (i.e., thegreatest common divisor of r and N equals 1).

Any GCL sequence has an ideal periodic autocorrelation function, i.e. itis a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence.

If the two GCL sequences c_(x)(k) and c_(y)(k) are defined by using thesame Zadoff-Chu sequence {a(k)} but different, arbitrary modulationsequences {b_(x)(k)} and {b_(y)(k)}, it can be shown (in a mannersimilar to what is disclosed in B. M. Popovic, “New Complex Space-TimeBlock Codes for Efficient Transmit Diversity,” IEEE 6th Int. Symp. onSpread-Spectrum Tech. & Appl (ISSSTA 2000), NJ, USA, pp. 132-136,September 2000.) that the periodic cross-correlation is zero for alltime shifts p in the delay zones

0<|p|<sm, sm<|p|<2sm, . . . , (m−1)sm,<|p|<sm².

Thus, if the above two modulation sequences are orthogonal, theresulting GCL sequences will be not just orthogonal, but also will havea zero-correlation zone of length sm−1.

Based on this property, the set of m zero correlation zone sequences canbe defined as the set of GCL sequences obtained by modulating a commonZadoff-Chu sequence {a(k)} with m different orthogonal modulationsequences {b_(i)(k)}, i=0, 1, 2, . . . , m−1, k=0, 1, 2, . . . , m−1.The periodic cross-correlation between any two sequences from the setwill be zero for all the delays between −sm and +sm.

The sequences from the set of zero-correlation zone sequences are usedas the synchronization signatures. Although the matched filters for suchsignatures actually calculate the aperiodic cross-correlations, it isexpected that the ideal periodic cross-correlation properties in thesearch window will be to a large extent preserved. The reason is thatfor delays in the search window that are much smaller than the length ofthe sequence, the sums for the aperiodic and periodic cross-correlationvalues only differ in a small number of terms. This expectation isconfirmed by numerical evaluations, as will be shown later.

For the GCL sequences, possible choices for the selection of orthogonalmodulation sequences would, for example, be either the sets of Hadamardsequences or Discrete Fourier Transform (DFT) sequences. The set of DFTsequences is defined asb _(i)(k)=W _(m) ^(ik) , i,k=0, 1, . . . , m−1,  (5)

while the set of Hadamard sequences is defined as the rows (or columnsor possibly both rows and columns) in a m×m Hadamard matrix, defined asfollows: A Hadamard matrix H_(m) of order m, consists of only 1s and −1sand has the property H_(m)H_(m) ^(T)=mI where I is the identity matrixand ^(T) denotes transpose. Hence, the Hadamard sequences areorthogonal. For m=2^(n), where n is a positive integer, Hadamardsequences can be defined as

$\begin{matrix}{{{b_{i}(k)} = \left( {- 1} \right)^{\sum\limits_{i = 0}^{m - 1}{i_{l} \cdot k_{l}}}},i,{k = 0},1,\ldots\mspace{14mu},{m - 1},} & (6)\end{matrix}$

where i_(l), k_(l) are the bits of the m-bits long binaryrepresentations of integers i and k.

The actual numbers m and N can be selected to fit into the requirementsof the Evolved UTRA. For a given length of the sequence, there is then atrade off between the length of the zero-correlation zone and the numberof signatures that can be provided.

For example, for a 1.25 MHz bandwidth in Evolved UTRA, an exemplary timeavailable for signature sequence transmission is 500 μs, and with aguard time of about 110 μs, the duration of the sequence is 390 μs.Assuming a sampling rate of, e.g., 1.024 MHz, it follows that the lengthof the sequences is N=400=sm².

The cell size is generally known, and thereby the maximum timedifference between signals from two mobile terminals in the cell (i.e.,the sum of the additional propagation times to and from the one mobileterminal relative to the other). Advantageously, the zero-correlationzone length is adapted to this time difference, i.e. to obtain a lowcorrelation for all possible time differences up to the maximum possibledifference. If, for example, the cell size is 14 km, the maximum traveltime for a signal corresponds to 96 symbols with the above presumptions.The low cross-correlation in this delay range will, according to theabove, be ensured if sm=100, so it follows that m=4, and s=25 (a largerm would result in a shorter, and thereby unsatisfactory,zero-correlation zone length). For simplicity, we choose q=0 in equation(4). It is to be understood, however, that other values of q may beused. A non-zero q will cause a shift in the sequence. Hence, there are4 different signature sequences of length 400.

In FIG. 2 is shown the absolute values of the autocorrelation andcross-correlation functions of the sequences with the received signal.

The amplitude of the aperiodic cross-correlation function

${{R_{xy}(p)} = {\sum\limits_{k = 0}^{N - 1 - p}{{c_{x}(k)}{c_{y}^{*}\left( {k + p} \right)}}}},$where p is the delay and “*” denotes complex conjugate, is shown in FIG.2 for the DFT-modulated GCL sequence, with N=400 (s=25 and m=4), andr=1.

A set of Hadamard-modulated GCL sequences has autocorrelation andcross-correlation functions similar to the ones shown in FIG. 2. Thepeaks of the cross-correlation functions are located near multiples ofsm=100. The peaks exhibit a certain broadening, i.e. the correlationvalues close to multiples of sm have considerable non-zero values, whichare not otherwise present for the periodic cross-correlation functions.However, for the given parameters, the cross-correlation functions donot exceed 20 for delays less than 96. Accordingly, for a cell of size14 km, only the portion of the plot up to p=96 is of interest, and inthis interval the result of the correlation is unambiguous. The portionof the plot in FIG. 2 showing delays from 0 to 100 is shown more indetail in FIG. 3.

The actual synchronization is performed by the base station using a setof matched filters to correlate the received signal with the signaturesequences in the set of signature sequences for all delays within thesearch window, and detecting a peak output from each matched filter. Athreshold is used to reduce the probability of false detection, i.e.,the threshold is set to a value such that when the received signal onlyconsists of noise it results in a detection with a certain probability,e.g. 0.0001.

The detected peak output from each filter is then used to estimate thetime of arrival, i.e. the delay, to synchronize the transmission fromthe mobile terminal.

The comparison signal in the base station may be non-periodic, i.e.,consist of only one period. Alternatively, this signal may be periodicor consist of one period plus a portion of a period on either or bothsides. If a periodic signal is used, the threshold must be increasedsince the probability of an erroneous detection increases. On the otherhand, the robustness is increased when more than one signature sequenceare present. Further, it is, of course, also possible to extend thesignature sequence transmitted by the mobile terminal a portion of aperiod on either or both sides of the sequence. The length of theadditional portion(s) may be determined by the time available fortransmitting the signature sequence.

In one embodiment of the present invention, all cells in a system areprovided with the same number of signature sequences, preferably thisnumber is selected based on the largest cell in the system. As isapparent, however, the specific signature sequences may vary from cellto cell. This has the advantage that when a mobile terminal is presentat the border between two cells, it can be determined which cell ittries to connect to. If neighbouring cells have the same set ofsignature sequences, two or more base stations may attempt to answer thecall from the mobile terminal. On the other hand, it may be determinedwhich base station provides the best signal quality, and thereby whichbase station should answer. As also is apparent from the above, however,it is also possible to have different sets of signature sequences indifferent cells. The various sets of signature sequences can easily beobtained by varying r. Which r value to use may be transmitted to themobile terminal, which thereby can produce the set of signaturesequences according to the equations above. Further, if the cell size issmaller, the number of signature sequences may be increased withmaintained sequence length. If, for example, the cell size is 7 km, thenumber of delay steps needed is only half of the above example.Accordingly, m can be set to 7 and s to 8. This will result in asignature sequence of length 392, and 7 signature sequences fulfillingthe requirements of the cell size, i.e. during sm−1=55 steps. In thisexample, the above guard time has been maintained. It is, however, alsopossible to reduce the guard time in smaller cells and thereby enablelonger signature sequences, and consequently also an increased number ofsequences.

The detection performances of these proposed signature sequences, orpreambles, have been evaluated by link-level simulations. The truncatedWCDMA RACH preamble has been used as a reference with modulatingHadamard sequences that are 4 bits long, instead of 16 bit longsequences, to keep the same number of signature sequences as for theproposed sequences. The number of receive antennas is two andcorrelations from the two antennas at the same delay are combinednon-coherently, i.e. the absolute values of the squared matched filteroutputs from the two antennas at the same delay are added. The number oftrials is 100000.

Two scenarios have been simulated. In both scenarios the detectorcorrelates the received signal with all possible signature sequences inthe search window. A threshold is set to give a false alarm probabilityof 0.0001 for a signature sequence at a single delay. Missed detectionis declared if the transmitted signature sequence is not detected.

In the first scenario, only one preamble is transmitted in atime-frequency resource for RACH. The delay is randomly distributedwithin the search window, i.e., in this example, ranging from 0 to 96samples, corresponding to randomly distributed mobiles in the cell.

In the second scenario, two or more different signature sequences fromthe same set are transmitted in the same time-frequency resource. Thesignal-to-noise ratio (SNR) of signature S1 is fixed (SNR=−15 dB) andthe other interfering signatures are transmitted with various poweroffsets to signature 1. However, all interfering signatures aretransmitted with the same power. All signatures are transmitted withindependent random delays within the search window. The probability ofmissed detection of the weaker signal, signature S1, is recorded. TheSIR is the ratio of the power of signature S1 to the power of any of theinterfering signatures.

Simulation results are shown for scenarios 1 and 2 in FIGS. 4 and 5,respectively. In FIG. 4, the probability of missed detection for onetransmitted sequence is shown, and in FIG. 5 the probability of misseddetection of a transmitted sequence in presence of another transmittedsequence is shown. From FIG. 4 it is clear that there is no differencein the probability of missed detection in the case without aninterfering sequence as compared to the prior art. Hence, in thissituation, the signature sequences according to the present inventionperform as well as the prior art sequences.

Regarding the second scenario, however, with two or more simultaneously,or substantially simultaneously transmitted sequences, the results shownin FIG. 5 clearly demonstrate significantly improved detectionperformance in the presence of one or several interfering sequences forthe set of sequences according to the present invention. For theproposed set of sequences, the detection performance does not changewith an increased number of interferers, not even for very low SIRvalues, whereas for the reference sequences, the performancedeteriorates substantially, both as the number of interferers increases,and with decreasing SIR. This substantial difference can, at leastpartially, be explained by the condition that when a strong signal and aweak signal are simultaneously present, parts of the stronger signalwill, during correlation, be interpreted as part of the weaker signal,with an incorrectly calculated delay as result. The use of signaturesequences according to the present invention has the advantage that, ascan be seen in FIG. 5, even if the signal level of one signaturesequence is strong while the signal level of a substantiallysimultaneous signature sequence is considerably weaker, the probabilityof a correct detection is substantially improved.

The lower probability of missed detection exhibited for the proposed setof sequences is due to the good cross-correlation properties of thezero-correlation zone sequences, and consequently, the present inventionprovides a substantial improvement as compared to the prior art.Further, this improvement of the detection probability by the use ofzero-correlation zone sequences can allow reduction of the transmittedpower for RACH preamble, thereby reducing the overall interference inthe system and increased battery life.

Further, in the above description the invention has been described asutilizing full zero-correlation zone sequences. It is, however, alsopossible to use truncated sequences, i.e, not all of thezero-correlation zone sequences are used. This will reduce the detectionprobability, however with the advantage that the freedom in selectingnumber of signature sequences for a particular signature lengthincreases. The truncation may vary with the cell size. In smaller cellsa larger truncation may be accepted with maintained satisfactoryperformance.

As has been disclosed above, the present invention has severaladvantages. There are, however, other characteristics that have to beconsidered in order for the system to operate properly. For example, ashas been mentioned above, it is important that the transmitted signaturesequence requires a low power amplifier back-off to allow for highaverage transmit power and hence good coverage. Two measures related tothe power back-off are the peak-to-average power ratio (PAPR) and thecubic metric (CM).

In the following, the impact of the present invention on these measureswill be disclosed.

Let z(t) be the normalized baseband signal, such that its expectationvalue E(|z(t)|²)=1.

The PAPR at the 99.9^(th) percentile is defined as the value x such thatthe probability that 10 log₁₀(|z(t)|²)<x equals 0.999.

The CM is defined asCM=[20 log₁₀((v_norm³)_(rms))−20 log₁₀((v_norm_ref³)_(rms))]/1.85  (7)

where

-   -   v_norm is the normalized voltage waveform of the input signal    -   v_norm_ref is the normalized voltage waveform of the reference        signal (12.2 kbps AMR Speech in WCDMA)

Table 1 lists the PAPR values at the 99.9^(th) percentile for areference WCDMA RACH preamble truncated to 400 samples with 4-bitHadamard modulating sequences, and for the GCL sequences with DFT andHadamard modulating sequences. Table 2 lists the corresponding CMvalues.

TABLE 1 PAPR (99.9th percentile) values Pulse-shaping GCL- filter WCDMAGCL-DFT Hadamard Sinc 3.9-5.9 dB 2.8 dB 4.5 dB Root-raised cosine,2.6-3.4 dB 3.0 dB 3.6 dB roll-off factor = 0.15

TABLE 2 Cubic metric values Pulse-shaping GCL filter WCDMA GCL-DFTHadamard Sinc 0.1-0.5 dB −0.6 dB 1.4 dB Root-raised cosine, −0.3 to −0.1dB −0.6 dB 1.1 dB roll-off factor = 0.15

In all cases the maximum PAPR value is given over all modulatingsequences. The range of values given for the WCDMA RACH preamble is overall scrambling codes. For the GCL sequences, the Zadoff-Chu sequencewith r=1 has been used. It is to be understood that this specificexample of the value of r only is exemplary. Two different pulse-shapingfilters are applied, a simple sinc filter and a root-raised cosinefilter with roll-off factor 0.15.

From the tables, it is clear that the DFT-modulated sequence has bothlower PAPR and lower cubic metric than the Hadamard-modulated GCLsequence. Furthermore, applying a root-raised cosine filter improvesneither PAPR nor the cubic metric of the DFT-modulated sequence.

Finally, the PAPR of the DFT-modulated GCL sequence is essentially asgood as the WCDMA sequences with a root-raised cosine filter, while thecubic metric is somewhat better than for the WCDMA sequences.Apparently, it is possible to find sets of zero-correlation zonesequences that allow for a low power back-off.

1. A method for performing an uplink synchronization between a firsttransceiver and a second transceiver within a cell in a multi-usercellular communication system, the method comprising: receiving, by thefirst transceiver, a signal from the second transceiver for the uplinksynchronization, the signal comprising a first signature sequence,wherein said first signature sequence is selected from a first set ofsignature sequences, and correlating, the received signal with a secondsignature sequence in the first transceiver, the second signaturesequence selected from a second set of signature sequences to estimate atime of arrival of said first signature sequence, so as to synchronize atransmission between the second transceiver and the first transceiver,wherein said first signature sequence is generated, at least in part,from a sequence with a zero-correlation zone.
 2. The method according toclaim 1, wherein correlating the received signal with the second signalsequence comprises: correlating one or more predetermined number ofdelays of the received signal with the second signature sequence forcalculating one or more amplitudes of aperiodic cross-correlation;determining a delay p corresponding to the maximum amplitude of the oneor more amplitudes; and estimating the time of arrival of said firstsignature sequence using the delay p, so as to synchronize thetransmission between the second transceiver and the first transceiver.3. The method according to claim 1, wherein the correlating the receivedsignal with the second signal sequence comprises: correlating one ormore predetermined number of delays of the received signal with thesecond signature sequence in the first transceiver using a set ofmatched filters, detecting a peak output from each matched filter, andusing the detected peak output from each filter to estimate the time ofarrival of the received signal, so as to synchronize the transmissionbetween the second transceiver and the first transceiver.
 4. The methodaccording to claim 1, wherein correlating the received signal with thesecond signal sequence comprises: correlating one or more predeterminednumber of delays of the received signal with each signature sequence insaid second set of signature sequences for calculating one or moreamplitudes of aperiodic cross-correlation; determining a delay pcorresponding to the maximum amplitude of the one or more amplitudes;and estimating the time of arrival of said first signature sequenceusing the delay p, so as to synchronize the transmission between thesecond transceiver and the first transceiver.
 5. The method according toclaim 1, wherein each signature sequence in the first and the second setof signature sequences is taken from a set of Generalized Chirp-Likesequences obtained by modulating a Zadoff-Chu sequence with anorthogonal set of complex sequences.
 6. The method according to claim 1,wherein said first signature sequence is randomly selected from thefirst set of signature sequences.
 7. The method according to claim 1,wherein said first set of signature sequences and said second set ofsignature sequences are the same set of signature sequences.
 8. Themethod according to claim 1, further comprising: transmitting areference representing a particular set of signature sequences to thesecond transceiver, and wherein the reference is used by the secondtransceiver to retrieve a signature sequence set scheme to be used. 9.The method according to claim 1, wherein said first signature sequenceis generated by truncating a sequence with a zero-correlation zone. 10.The method according to claim 1, further comprising: performing, by saidsecond transceiver, a downlink synchronization with said firsttransceiver prior to the uplink synchronization.
 11. The methodaccording to claim 1, wherein the second set of signature sequencescomprises one or more signature sequences of the first set of signaturesequences.
 12. The method according to claim 1, wherein the firstsignature sequence is used as a preamble portion of a signal transmittedby a mobile terminal to a base station over a random access channel. 13.The method according to claim 1, wherein the first signature sequence isused as a portion of a synchronization signal transmitted by a mobileterminal to a base station.
 14. The method according to claim 2, whereinthe zero-correlation zone of said first signature sequence has a lengththat substantially corresponds to a maximum expected delay of atransmission from the second transceiver to the first transceiver. 15.The method according to claim 5, wherein the orthogonal set of complexsequences is one of the following: a set of rows and/or columns of adiscrete Fourier transform matrix, and a set of rows and/or columns in aHadamard matrix.
 16. The method according to claim 14, wherein a cellsize is used to determine the maximum delay in said correlating stepand/or the maximum expected delay of the transmission from the secondtransceiver to the first transceiver.
 17. An apparatus, comprising: atransceiver, configured to receive a signal from another transceiver forperforming an uplink synchronization, said signal comprising a firstsignature sequence, wherein said first signature sequence is selectedfrom a first set of signature sequences, and a correlator configured tocorrelate the received signal with a second signature sequence selectedfrom a second set of signature sequences to estimate a time of arrivalof said first signature sequence, so as to synchronize a transmissionbetween the other transceiver and the transceiver of the apparatus,wherein said first signature sequence is generated, at least in part,from a sequence with a zero-correlation zone.
 18. The apparatusaccording to claim 17, wherein said correlator is further configured tocorrelate one or more predetermined number of delays of the receivedsignal with the second signature sequence for calculating one or moreamplitudes of aperiodic cross-correlation, determine a delay pcorresponding to the maximum amplitude of the one or more amplitudes,and estimate the time of arrival of said first signature sequence usingthe delay p, so as to synchronize the transmission between the othertransceiver and the transceiver of the apparatus.
 19. The apparatusaccording to claim 17, wherein the correlator is further configured to:correlate one or more predetermined number of delays of the receivedsignal with the second signature sequence using a set of matchedfilters, detect a peak output from each matched filter, and use thedetected peak output from each filter to estimate the time of arrival ofthe received signal, so as to synchronize the transmission between theother transceiver and the transceiver of the apparatus.
 20. Theapparatus according to claim 17, wherein the correlator is furtherconfigured to: correlate one or more predetermined number of delays ofthe received signal with each signature sequence in said second set ofsignature sequences calculating one or more amplitudes of aperiodiccross-correlation, determine a delay p corresponding to the maximumamplitude of the one or more amplitudes, and estimate the time ofarrival of said first signature sequence using the delay p, so as tosynchronize the transmission between the other transceiver and thetransceiver of the apparatus.
 21. The apparatus according to claim 17,wherein said first set of signature sequences and said second set ofsignature sequences are the same set of signature sequences.
 22. Theapparatus according to claim 17, wherein the apparatus is a basestation.
 23. The apparatus according to claim 17, wherein the second setof signature sequences comprises one or more signature sequences of thefirst set of signature sequences.
 24. The apparatus according to claim17, wherein the first signature sequence is used as a preamble portionof a signal transmitted by a mobile terminal to a base station over arandom access channel.
 25. The apparatus according to claim 17, whereinthe first signature sequence is used as a portion of a synchronizationsignal transmitted by a mobile terminal to a base station.
 26. Theapparatus according to claim 18, wherein the zero-correlation zone ofsaid first signature sequence has a length that substantiallycorresponds to a maximum expected delay of a transmission from the othertransceiver to the transceiver of the apparatus.
 27. The apparatusaccording to claim 26, wherein a cell size is used to determine themaximum delay in said correlating step and/or the maximum expected delayof a transmission from the other transceiver to the transceiver of theapparatus.
 28. An apparatus within a cell of a multi-user cellularcommunication system, comprising: a unit configured to select a firstsignature sequence from a first set of signature sequences, atransmitter configured to transmit a signal for an uplinksynchronization between the apparatus and a receiver, said signalcomprising the first signature sequence, so as to enable the receiver toestimate the time of arrival of said first signature sequence bycorrelating the signal with a second signature sequence selected from asecond set of signature sequences, wherein said first signature sequenceis generated, at least in part, from a sequence with a zero-correlationzone.
 29. The apparatus according to claim 28, wherein the apparatus isa mobile terminal.
 30. The apparatus according to claim 28, wherein thefirst signature sequence is used as a preamble portion of a signaltransmitted by a mobile terminal to a base station over a random accesschannel.
 31. The apparatus according to claim 28, wherein the firstsignature sequence is used as a portion of a synchronization signaltransmitted by a mobile terminal to a base station.